Negotiations continue between the World Health Organization and China over requests for the Chinese to share samples of H5N1 bird flu. International experts complain that China has not cooperated fully in the fight against the virus, and they fear this could hamper the global effort to prevent a pandemic.

For months, the World Health Organization has expressed frustration over China's failure to hand over tissue samples from animals that died of the H5N1 virus in the country in recent months.

WHO officials say Chinese laboratories have shared samples from human cases and have - in some instances - passed on data about the virus found in animals. However, scientists say that information amounts to only bits and pieces, and does not provide them with enough of a view to see how much the virus may be mutating.

Speaking in Beijing last month, Shigeru Omi, the WHO's top official for the Asia Pacific region, said it is essential that nations cooperate in the effort to develop vaccines and other prevention measures. He reminded the Chinese that they also have gained from the system of international cooperation.

"WHO provided [the] seed virus to all the manufacturers, international level manufacturers. This seed virus was derived from Vietnam," he said. "China received it. China benefited from this international collaboration."

The WHO says the virus is known to have infected more than 150 people, killing at least 80 people in the past two years, most of them in East Asia. Most human victims caught the virus from infected poultry and governments have killed tens of millions of chickens, ducks and other birds to contain the spread of H5N1.

Along with Vietnam, Indonesia and Thailand have been quick to share animal virus samples, allowing scientists to track H5N1 and hunt for signs of a mutation, especially a change that could make the virus easily transmissible from human to human. Scientists say they have not been getting the same level of cooperation from China.

Chinese officials have tried to counter the criticism by saying Beijing has cooperated with the international community by hosting conferences on avian influenza and sharing other information.

At an international donors conference in Beijing last week, Premier Wen Jiabao repeated his government's assertions that it has disclosed the information the WHO and others have requested.

The Chinese leader said China has been prompt and transparent in keeping international organizations and countries informed on the spread of the virus among birds and humans in his country.

WHO officials and observers say they see no signs of a cover-up like the one of 2003, when Chinese authorities failed to report early cases of SARS. Still, officials are at a loss to explain why China has been slow to hand over samples.

Dr. Juan Lubroth is the chief of infectious diseases at the veterinary service of the U.N.'s Food and Agriculture Organization in Rome. He says China's hesitation may be linked to the commercial benefits it may gain by having a monopoly on the samples. He says Chinese scientists also may be concerned about not getting credit for their work.

"Some Chinese scientists probably feel that they have not received the justified contribution to their scientific publications or scientific merit from the peer review journals that they think that they deserve," said Lubroth. "Some of their work has been scooped by others."

Arnold Monto is an epidemiologist at the University of Michigan's school of public health who has been following the spread of H5N1 bird flu. He says that whatever the cause of their hesitation, Chinese leaders should consider the long-term benefits of sharing information and tissue samples - if only because it helps ensure that other countries share information with China.

"It is to China's advantage to have these specimens considered, because if China is not handing over specimens or is not sharing their specimens, other countries could do it [not share information]," said Monto. "This could be dangerous in the long run to China because, with birds flying from place to place, with chickens being shipped from place to place, there's no telling where important virus variants may originate."

With no international laws binding China to comply with requests to share samples and information, all scientists can do is continue to urge the Chinese to be more forthcoming.

The WHO and the FAO have been negotiating with China's Ministry of Agriculture on the issue. A WHO spokesman this week said the government has responded to a request for better sharing of animal virus samples, and a dialogue is under way on which specific samples China will share.

At last week's pledging conference, China joined nearly 90 nations in reaching a broad agreement that includes a commitment to share information and other resources to battle bird flu.

The conference raised nearly $2 billion for global bird flu prevention efforts - the largest donation coming from the United States. Following up on a commitment to battle the spread of H5N1, the Bush administration pledged $334 million to health programs - mainly in East Asia and Africa.